Modern mobile telecommunications standards continue to demand increasingly greater rates of data exchange (data rates). One way to achieve a high data rate in a mobile device is through the use of carrier aggregation. Carrier aggregation allows a single mobile device to aggregate bandwidth across one or more operating bands in the wireless spectrum. The increased bandwidth achieved as a result of carrier aggregation allows a mobile device to obtain higher data rates than have previously been available.
FIG. 1 shows a table describing a number of wireless communications bands in the wireless spectrum. One or more of the wireless communications bands may be used, for example, in a CDMA, GSM, LTE, or LTE-advanced equipped mobile device. The first column indicates the operating band number for each one of the operating bands. The second and third columns indicate the uplink and downlink frequency bands for each one of the operating bands, respectively. Finally, the fourth column indicates the duplex mode for each one of the operating bands. In non-carrier aggregation configurations, a mobile device will generally communicate using a single portion of the uplink or downlink frequency bands within a single operating band. In carrier aggregation applications, however, a mobile device may aggregate bandwidth across a single operating band or multiple operating bands in order to increase the data rate of the device.
FIG. 2A shows a diagram representing a conventional, non-carrier aggregation configuration for a mobile device. In the conventional configuration, a mobile device communicates using a single portion of the wireless spectrum 10 within a single operating band 12. Under the conventional approach, the data rate of the mobile device is constrained by the limited available bandwidth.
FIGS. 2B-2D show diagrams representing a variety of carrier aggregation configurations for a mobile device. FIG. 2B shows an example of contiguous, intra-band carrier aggregation, in which the aggregated portions of the wireless spectrum 14A and 14B are located directly adjacent to one another and are in the same operating band 16. FIG. 2C shows an example of non-contiguous intra-band carrier aggregation, in which the aggregated portions of the wireless spectrum 18A and 18B are located within the same operating band 20, but are not directly adjacent to one another. Finally, FIG. 2D shows an example of inter-band carrier aggregation, in which the aggregated portions of the wireless spectrum 22A and 22B are located in different operating bands 24, 26. It is advantageous for a modern mobile device to support each one of the previously described carrier aggregation configurations.
The use of carrier aggregation may pose unique problems for the front end circuitry in a mobile device. For instance, the use of carrier aggregation across certain operating bands may cause undesirable interference between transmit and receive circuitry in a mobile device front end that renders the mobile device unusable in these operating bands. Accordingly, the use of carrier aggregation in a mobile device may complicate the design of the front-end circuitry.
FIG. 3 shows conventional front end circuitry 28 for use in a mobile terminal. The conventional front end circuitry 28 includes antenna switching circuitry 30, a diplexer 32, and an antenna 34. The antenna switching circuitry 30 includes high band switching circuitry 36 and low band switching circuitry 38. The high band switching circuitry 36 is adapted to couple one of a plurality of high band RF front end ports 40 to the antenna 34 through the diplexer 32. The low band switching circuitry 38 is adapted to couple one of a plurality of low band RF front end ports 42 to the antenna 34 through the diplexer 32. The diplexer 32 includes a low band port 46 coupled to the low band switching circuitry 38, a high band port 44 coupled to the high band switching circuitry 36, and an antenna port 48 coupled to the antenna 34. The diplexer 32 is adapted to pass high band signals falling within a high pass band between the high band port 44 and the antenna port 48, pass low band signals falling within a low pass band between the low band port 46 and the antenna port 48, and attenuate signals outside of the respective high and low pass bands. Although effective at selectively placing the antenna 34 in communication with the appropriate RF front end port, in certain carrier aggregation configurations, the conventional front end circuitry 28 shown in FIG. 3 may suffer from undesirable interference between the transmit and receive signal paths within the high band switching circuitry 36 and the low band switching circuitry 38.
The antenna switching circuitry 30 may comprise a plurality of transistors and other assorted passive components. As is well known in the art, non-linearity of the transistors and other passive components within the antenna switching circuitry 30 may generate harmonic distortion about a passing signal. In certain carrier aggregation applications, the generated harmonic distortion can cause desensitization of receive circuitry in the conventional front end circuitry 28 illustrated in FIG. 3. For example, in a carrier aggregation configuration using bands 3 and 8 (CA 3-8), the conventional front end circuitry 28 will couple one of the low band RF front end ports 42 corresponding with the band 8 TX port to the low band port 46 of the diplexer 32 in order to transmit a carrier signal between 880-915 MHz. As the carrier signal passes through the low band switching circuitry 38, harmonic distortion is generated. The carrier signal and harmonic distortion travel through the diplexer 32, where the carrier signal is passed from the low band port 46 to the antenna port 48.
Although the carrier signal is at least partially filtered when passing through the diplexer 32, conventional diplexer circuitry may only partially attenuate harmonic distortion about the carrier signal. Because at least a portion of the second harmonic of the band 8 uplink signal (1760-1830 MHz) falls within the band 3 downlink signal (1805-1880 MHz), components of the harmonic distortion about the carrier signal are within the high pass band of the diplexer 32, and a portion of the harmonic distortion will be delivered to the high band switching circuitry 36. Further, because the conventional front end circuitry 28 is configured to simultaneously transmit on band 8 and receive on band 3 in a CA 3-8 configuration, one of the high band RF front end ports 40 corresponding with the band 3 will be coupled to the diplexer 32 through the high band switching circuitry 36. Accordingly, a portion of the high amplitude carrier signal about the second harmonic will be delivered to the band 3 receive circuitry, where it will cause desensitization. Additionally, the harmonic distortion in the carrier signal will be presented to the antenna 34, thereby degrading the quality of the transmission signal. As a result of the desensitization of the receive circuitry and the degraded quality of the transmission signal, the performance of the conventional front end circuitry 28 shown in FIG. 3 may suffer in a CA 3-8 configuration.
As an additional example, the conventional front end circuitry 28 will also experience problems in carrier aggregation applications using bands 4 and 17 (CA 4-17), because the third harmonic of band 17 (2112-2148 MHz) falls within band 4 (2110-2155 MHz). The problem with the conventional front end circuitry 28 may occur in any carrier aggregation configuration using operating bands in which the harmonic components of the carrier signal fall within the frequency band of the receive signal. The limited combination of operating bands usable in a carrier aggregation configuration by the conventional front end circuitry 28 may impede the performance and versatility of the mobile device. Accordingly, front end circuitry for a mobile device is needed that is capable of reducing or eliminating undesirable interference between transmit and receive paths in carrier aggregation applications.
FIG. 4 shows a conventional diplexer 50 for use in the conventional front end circuitry 28 shown in FIG. 3. The conventional diplexer 50 includes an antenna port 52, a low band port 54, a high band port 56, a high pass filter 58, and a low pass filter 60. The high pass filter 58 is adapted to pass high band signals about a high pass band between the antenna port 52 and the high band port 56, while attenuating signals outside of the high pass band. The low pass filter 60 is adapted to pass low band signals about a low pass band between the antenna port 52 and the low band port 54, while attenuating signals outside of the low pass band.
The conventional diplexer 50 allows a mobile terminal to transmit and receive a high band signal and a low band signal simultaneously, thereby increasing the data rate of the mobile device. Although effective at separating low and high band signals, the conventional diplexer 50 may only partially attenuate harmonic distortion about a passing signal. In certain carrier aggregation applications, the limited isolation characteristics of the conventional diplexer 50 may degrade the performance of a mobile device in which it is incorporated. Carrier aggregation applications may demand more precise control over the high and low pass bands, greater stop band attenuation, and lower insertion loss. To achieve the desired pass and stop bands, a seventh or eighth order Butterworth response may be required according to the conventional design. Such a high order filter is complex to implement, and further introduces a high amount of insertion loss into the signal path to the antenna.
FIG. 5 shows a graph of the filter response for a diplexer using a high order number of filters. The x-axis 62 of the graph represents the frequency of a signal passed through the diplexer, while the y-axis 64 of the graph represents the attenuation associated with the signal path. As shown in FIG. 6, a low band signal path 66 provides low attenuation at low frequencies, while providing high attenuation at high frequencies. Further, a high band signal path 68 provides low attenuation at high frequencies while providing high attenuation at low frequencies. As discussed above, high order filters may be required to meet the demanding standards of carrier aggregation applications. Even with a high order filter, however, conventional diplexer circuitry may not provide adequate isolation characteristics 70 between the low band signal path 66 and the high band signal path 68. Further, insertion loss 72 associated with a high order diplexer may significantly degrade the performance of a mobile terminal in which it is integrated. Accordingly, a diplexer is needed that is capable of providing the necessary pass bands for the high band and low band signals while maintaining desirable stop band attenuation and insertion loss for carrier aggregation applications.